Interface Disorder Research Articles

Diamagnetic shift and localization of excitons in disordered quantum wells

Calculations of exciton absorption spectra and wave functions in quantum wells with alloy and interface disorder under the influence of a perpendicular magnetic field are presented. The four-coordinate Schrödinger equation is solved in electron–hole in-plane coordinates thus going beyond the standard factorization ansatz. It is shown that the diamagnetic shift varies remarkably among different states and is not monotonic in energy, which is in agreement with recent experiments. The stronger the localization of the wave function the smaller the diamagnetic shift. Therefore, the diamagnetic shift analysis gives additional information about the local potential landscape in quantum wells.

Quasi-ballistic transport of excitons in quantum wells

The transport of quantum-well excitons is investigated in the model system ZnSe on the length scale of the light wavelength and on the timescale of the first inelastic scattering processes. The experiments are based on confocal microscopy including a solid immersion lens in the setup to achieve a spatial resolution of 250 nm.The excitons propagate in the quantum-well plane quasi-ballistically for several 100 nm only affected by elastic scattering due to interface disorder. The first inelastic scattering process, which destroys the coherence of the exciton wave function, is the interaction with acoustic phonons after about 30 ps. This process results in the reversal of the excitonic propagation direction which is directly reflected by a spatial oscillation in the temporal evolution of the photoluminescence spot. From the oscillation one can deduce simultaneously the coherence time and length of the excitonic transport. We describe the transition to diffusive exciton transport with increasing temperature and the influence of strong interface disorder.

Interface disorder and inhomogeneous broadening of quantum well excitons: Do narrow lines always imply high-quality interfaces?

It is a commonly assumed that narrow lines in absorption or luminescence of quantum well excitons at low temperatures indicates high quality of quantum well interfaces. We show, that at least for narrow quantum wells, this is not always the case. Correlations between morphological fluctuations of two interfaces confining a quantum well, which were neglected in previous studies of exciton line shape, strongly suppress an inhomogeneous broadening due to interface disorder.

Antiferromagnetism at the YBa2Cu3O7/La2/3Ca1/3MnO3 interface

The magnetic properties of a series of YBa2Cu3O7−x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behavior. However, field-cooled hysteresis loops at a low temperatures show the unexpected existence of exchange bias effect associated with the existence of ferromagnetic/antiferromagnetic (AF) interfaces. The blocking temperature (TB) is found to be thickness dependent and the exchange bias field (HEB) is found to be inversely proportional to the ferromagnetic layer thickness, as expected. The presence of an AF material is probably associated with interface disorder and Mn valence shift toward Mn4+.

Interface disorder and transport properties in HTC/CMR superlattices

The physical properties of superlattices are affected by interface disorder, like roughness and interdiffusion. X-ray diffraction allows its measurement through modeling and structure refinement. The high- T c RBa 2Cu 3O 7 (RBCO) and colossal magnetoresistance La x A 1−xMnO 3 (LAMO) perovskites are interesting superlattice partners given their similar lattice parameters and because the combination of magnetic and superconducting properties is interesting for both basic and applied research. We have investigated the structural and transport properties of YBCO/La 2/3Ca 1/3MnO 3 and GdBCO/La 0.6Sr 0.04MnO 3 superlattices grown by sputtering on (1 0 0)MgO. We find a roughness of 1 RBCO unit cell and a 30% interdiffusion in the same length from the interfaces for all samples. The superconducting behavior is found strongly dependent on the LAMO layer thickness.

Interface Effects in Perovskite Superlattices

The effect of interface disorder in perovskite superlattices, either with the substrate or between layers dominates the physics of the material, even when the lattice parameter of the component materials differs in less than 1%. Unexpected behavior emerges, like exchange bias in a system where no antiferromagnetic material has been included in the superlattice design.

Tunneling characteristics and low-frequency noise of high-Tcsuperconductor/noble-metal junctions

We report extensive measurements of transport characteristics and low-frequency resistance noise of c-axis yttrium-barium-copper-oxide (YBCO)/Au junctions. The dominant conduction mechanism is tunneling at low temperatures. The conductance characteristic is asymmetric, and the conductance minimum occurs at a nonzero voltage. These features can be qualitatively explained by modeling the YBCO/Au interface with a Schottky barrier. The model shows that the YBCO surface behaves like a p-type degenerate semiconductor, with a Fermi degeneracy of about 0.1 eV. The barrier height is approximately 1.0 eV. We present evidence that interface states and disorder play an important role in determining the conductance characteristics. Low-frequency noise measurements of these junctions reveal that junction noise is dominated by resistance fluctuations with a 1/f-like power spectrum over a wide range of temperature and bias voltage. For temperatures between 4.2 and 77 K, the junction noise can be parameterized in terms of a normalized resistance fluctuation: δR/R≃6.3 ×10 - 4 /f, in units of Hz - 1 / 2 , where f is the center frequency of the measurement bandwidth. At f = 10 Hz, for example, it is 2 × 10 - 4 Hz - 1 / 2 . This noise figure should prove to be useful for engineering design of high-T c electronics. A more detailed analysis shows that at low temperatures the noise spectrum is characterized by random telegraph signals withe a Lorentzian power spectrum, which can have a distribution of corner frequencies that mimics a 1/f dependence. The random telegraph signals provide evidence for the existence of localized states.

Effects of interface disorder on transmission probability in magnetic multilayer

We present a study of conductance through disordered Fe-Cr[100] and Co-Cu[100] interfaces within a realistic tight-binding model. In our model, the specular and diffuse parts of the conductance are treated on an equal footing. We observe a substantial increase in conductance due to disorder in the channels that are strongly reflected by pure interfaces. We also show how this phenomenon affects the giant magnetoresistance. In addition, we find that the series resistor model may significantly overestimate the resistance if the thickness of the disordered layer is less than several times the electron mean free path.

Spectral Zeeman hole burning in a quantum dot ensemble

Spectral hole burning experiments based on high-resolution magneto-CW four-wave mixing in the phase conjugate geometry show that both the spectral and spatial diffusion of excitons is negligible in the GaAs quantum dot system formed by interface disorder. Due to the strong Coulomb coupling, Zeeman spectral hole burning is observed when we resonantly excite one of the two orthogonally polarized excitons confined to each dot by two copolarized and frequency-degenerate fields and then probe the system by a third beam that is of opposite polarization and a Zeeman splitting away (in frequency). The experiments allow for a measure of the exciton decay rates and g factor.

Femtosecond Pump–Probe Spectroscopy of GaAs Crescent Quantum Wires

We have investigated the ultrafast carrier dynamics in high-optical-quality V-grooved GaAs/AlGaAs quantum wires using pump-probe spectroscopy. It is found that, with resonant excitation, a blue shift due to exciton–exciton interaction occurs within 0.3 ps followed by a redshift due to exciton relaxation. Under nonresonant excitation, the redshift occurs due to the phonon emission during exciton formation. As a result, the exciton formation time and the exciton lifetime in the quantum wires are found to be 5 and 110 ps, respectively. This exciton lifetime corresponds to the free-exciton lifetime close to the theoretical intrinsic lifetime of one-dimensional excitons and proves that our quantum wires have little interface disorder and have high optical quality.

Design of high electron mobility devices with composite nitride channels

Field effect transistors based on a GaN/AlGaN system have shown remarkable performance characteristics in a wide range of device applications. However, due to the large effective mass of GaN, the mobility in the channel is small. In this work, we consider a GaN/AlGaN structure with a thin InN channel of the order of a few monolayers. We find that mobility in the channel can improve considerably while breakdown characteristics are not expected to suffer. Mobilities of ≃2500 cm2 (V s)−1 are predicted along with high sheet charges for low interface disorder. Good agreement with experimental results is observed for higher degrees of disorder within the model. At higher electric fields, we find that most electron population transfers to higher valleys or other subbands that lie in AlGaN or GaN. We also compare the low-field mobility-charge product for this structure with the conventional AlGaN/GaN structure and find that the two values are similar.

MOSFET degradation kinetics and its simulation

In this work, the time-dependence of Si/SiO/sub 2/ interface trap formation is considered by solving an improved set of Si-H defect kinetics equations that take into account interface disorder and the Si-H bond activation energy evolution as the bonds are broken. This model is applied to the simulation of metal oxide semiconductor field effect transistor (MOSFET) high field and hot carrier degradation, and then verified with various experimental data. An estimation of the potential barrier of the Si/SiO/sub 2/ interface is given.

Study of the misfit dislocations in semiconductor heterostructures by density functional TB molecular dynamics and path probability methods

The atomic and electronic structures of semiconductor heterostructures are studied using the density-functional tight-binding molecular dynamics simulations. Atomic structures of misfit dislocations both edge type 1/2 〈110〉 (001) and 60° dislocations in the semiconductor heterostructures, like Ge/Si(001), InAs/GaAs and InP/GaAs(001) systems are studied by using order of N [O(N)] calculational method. The path probability method in the statistical physics is used to study the influence of the interface disorder on the atomistic properties of the semiconductor heterostructures. It is shown that the compositional intermixing influences quite significantly on the electronic and thermodynamic properties of semiconductor heterostructures, especially on the critical layer thickness hc for the generation of misfit dislocations.

Spin injection through an Fe/InAs interface

The spin dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarization between 98% and 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry breaking arising from interface disorder reduces the spin asymmetry substantially, and we conclude that efficient spin injection from Fe into InAs can only be realized using high-quality epitaxial interfaces.

Spin-injection through an Fe/InAs interface

The spin-dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first-principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarisation between 98 an 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry-breaking arising from interface disorder reduces the spin asymmetry substantially and we conclude that efficient spin injection from Fe into InAs can only be realized using high quality epitaxial interfaces.

Design of composite channels for optimized transport in nitride devices

Heterostructure field effect transistors (HFETs) based on AlGaN / GaN structures have shown good performance as high power and high frequency devices. Theoretical simulations of transport in short channel HFETs show that there are several areas where considerable improvements in mobility, etc. can be made if thin composite structures (as considered in this work) can be utilized. We examine transport in a metal / AlGaN / InN / GaN composite structure. The InN region is very thin (∼ 15 Å) and is introduced to improve the low field transport without significantly impacting the device breakdown properties. Our model is capable of examining any other type of composite structure as well. Our simulation method consists of charge control solution of the heterostructure wave functions, followed by Monte Carlo simulation of scattering and free flight events. We present comparison of results on i) metal / AlGaN / GaN structure, and ii) a metal / AlGaN / GaN structure with a thin InN channel of the order of a few mono-layers. We find that mobility in the channel can improve considerably with very little effect on the mobility - charge product. Indeed, the charge density induced in the thin InN channel region is ∼ 1013cm∼2. While the peak velocities in a metal / AlGaN / InN / GaN structure exhibit an increase of nearly 30% over the values in metal / AlGaN / GaN structures, the low field mobilities are also increased. Low-field mobilities of ∼ 2500cm2(V ·s)∼ are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface disorder (∼ 70Å), we have found good agreement with experimental Hall mobility data for similar structures. At higher electric fields, we find that most electron population transfers to higher valleys or other sub-bands that lie in AlGaN or GaN. This ensures that high field breakdown of low band gap InN layer is also suppressed.

Pathologic myopia: where are we now?

PURPOSE: To describe current concepts and available treatments for pathologic myopia. DESIGN: Review of experimental and clinical studies. METHODS: The demography, natural history, medical and surgical treatments for choroidal neovascular membrane, vitreoretinal interface disorders and future strategies for pathologic myopia are reviewed. RESULTS: Several medical and surgical modalities are currently available to treat various complications of pathologic myopia. Macular translocation appears to stabilize or improve visual function in many eyes with choroidal neovascularization. CONCLUSION: Newer strategies are emerging to better ameliorate or prevent the complications of pathologic myopia.

MECHANICAL AND ELECTRONIC PROPERTIES OF STRAINED LAYER SUPERLATTICES STUDIED BY DENSITY FUNCTIONAL TB AND PATH PROBABILITY METHODS

The atomic and electronic structures of semiconductor heterostructures, including steps, misfit dislocations and interface disorder, are studied by using the density-functional tight-binding (TB) method. Atomic structures of misfit dislocations both edge type 1/2 <110> (001) and 60° dislocations in the semiconductor heterostructures, like Si-Ge superlattices and GaAs/Si, InAs/GaAs(001), InP/GaAs(001) systems, are studied by using order of N [O(N)] calculational method. The path probability method (PPM) in the statistical physics is used to study the influence of the interface disorder on the electronic properties of the semiconductor heterostructures. It is shown that the junction relaxation influences quite significantly the electronic and mechanical properties of semiconductor heterostructures. The critical layer thickness hc for the generation of misfit dislocations depends significantly on the interface disorder (increase of hc) at the semiconductor heterostructures.

Optical studies of the correlation between interface disorder and the photoluminescence line shape in GaAs/InGaP quantum wells

Photoluminescence (PL) and excitation PL measurements have been performed at different temperatures in a number of lattice-matched GaAs/In0.49Ga0.51P quantum wells, where the uctuations of the potential energy are comparable with the thermal energy of the photocreated carriers. Two samples with different well widths allow to observe a series of anomalous e ects in their optical response. The observed effects are related to the disorder in the interface, characterizing uctuations in the confinement potential energy. It is proposed that the carrier relaxation processes occur either at the local minima or at the absolute minimum of the confinement potential, depending on the ratio of the thermal energy and the magnitude of the potential fluctuations.

Structure and stability of hcp bulk and nano-precipitated Ag2Al

We study the short- and long- range chemical ordering in hcp bulk Ag2Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure of bulk Ag2Al is that proposed from X-ray data, but there are several competing metastable hcp structures. Our results provide the structural and thermodynamic properties of Ag2Al, with good agreement to experimental short-range order data (after reprocessing our data according to experimental procedure). We also discuss the influence of the Al:Ag2Al interface, coherency strain, and off-stoichiometric disorder on the structure of metastable Ag2Al γ′ nano-precipitates in an fcc Al matrix. We show that γ′ precipitates are off-stoichiometric and we provide a new structure that reproduces the observed transmission electron microscopy image while allowing for a distribution of Ag concentrations amongst the precipitates.